1. Field of the Invention
The present invention relates to a process for the preparation of a hydrogel, and more particularly to a process for the preparation of hydrogel having properties superior to those of the conventional hydrogels from a polyvinyl alcohol.
2. Prior Art
A variety of gel compositions have hitherto been proposed to be used as cooling media and some of them are applied for practical uses. Such a gel is referred to as a cooling gel, chilling gel, coldness-keeping gel, coldness-keeping heat transfer medium, colloidal coolant, coldness-keeping means, coolant composition of Ice-non (Trade Name), i.e. ice pillow. However, these known coolant gels are not satisfactory in properties or involve troublesome or difficult steps in preparation, as will be summarized below.
The easiest and well-known process for the preparation of a water-containing coolant gel is the preparation of agar gel. This process comprises the step of dissolving agar in boiling water or hot water of 80.degree. to 94.degree. C. to obtain a solution containing not less than 0.1 wt %, for example 1 to 10 wt % of agar and the step of allowing the solution to be cooled to room temperature, whereby a water-containing gel (hydrogel) having a water content of about 90 to 99 wt % is easily prepared. However, when the thus prepared agar gel is used as a substituting material for ice contained in an ice pillow or an ice bag after being cooled in an ice box of a refrigerator, the agar gel is poor in elasticity and too crumbly to provide a user with a comfortable touch or feel but gives a strange and uncomfortable feeling with additional disadvantage of collapse. A further disadvantage of the agar gel is that it is often frozen to form a rigid mass in a cooling box of a refrigerator, the rigid mass being objectionable for usage as a substituent for the ice bag. Although the freezing temperature of agar may be lowered by immersing agar in an antifreezing solution, such as ethylene glycol or propylene glycol, or by cooling a hot aqueous solution containing both agar and an antifreezing solution, the thus obtained gel is also crumbly and weak and, therefore, improper for use as a coldness-keeping gel in an ice pillow or the like.
It is already well-known that gelation occurs instantaneously when boric acid (or an aqueous solution of boric acid) or a borax (or an aqueous solution of borax) is added to an aqueous solution of polyvinyl alcohol. However, the gel thus obtained is so soft and weak that it is readily torn to pieces simply by pinching by fingers. Another disadvantage of this gel is the syneresis phenomenon, i.e. separation of contained water, during repeated uses (see Japanese patent publication No. 11210/1970).
In order to avoid hardening of a gel of a polyvinyl alcohol-borax system due to congealing of water content in the gel when the gel is cooled in an ice-box of a cooling box or refrigerator, there has been proposed a method in which an aqueous solution of polyvinyl alcohol is added with any of monohydric alcohols, polyhydric alcohols, glucose or sucrose followed by the addition of borax to be gelatinized. However, a gel prepared in accordance with this proposal becomes weaker with additional disadvantage that syneresis appears during repeated uses (see Japanese patent publication No. 19602/1971).
An alternate method for lowering the freezing point of the polyvinyl alcohol-borax system gel has been conceived, wherein the gel is immersed in an antifreezing solution, such as ethylene glycol or propylene glycol. However, the gel becomes too soft by the inclusion of ethylene glycol or propylene glycol to retain its shape. The polyvinyl alcohol-borax system gel has disadvantage in that it is apt to collapse by the action of other antifreezing solutions, such as methanol, ethanol and acetone.
After making researches for the development of a water-insoluble gel which is excellent in mechanical properties and hardly frozen in an ice box of a refrigerator by the use of polyvinyl alcohol in a low cost and in a stable condition, we have developed a process for the preparation of a hydrogel by cooling to solidify polyvinyl alcohol followed by dehydrating the solidified mass in vacuum and filed a pending U.S. patent application Ser. No. 344,006 on Jan. 29, 1982. We have filed another U.S. patent application Ser. No. 400,821 filed on July 22, 1982, relating to a process for the preparation of a gel for use as a cooling medium, the process being developed based on the finding that a solid gel which is highly elastic, excellent in softness and hardly to be frozen can be prepared by cooling to solidify an aqueous solution or an aqueous suspension containing a polyvinyl alcohol and a water-soluble polyhydric alcohol followed by dehydrating the solidified mass in vacuum. Based on the same finding, we have developed a gel for cooling a patient's head for preventing alopecia due to a harmful side effect of carcinostatics, and filed another patent application which is now pending as Japanese patent appln. No. 77502/1982.
The process for the preparation of a gel for use as a cooling medium, provided by our formerly-made proposals, comprises the steps of cooling to solidify an aqueous solution containing 2.5 to 10 wt % of a polyvinyl alcohol having a degree of hydrolysis of not less than 95 mol % and an average polymerization degree of not less than 1500 and 20 to 80 wt % of a water-soluble polyhydric alcohol having 2 to 6 hydroxyl groups to a temperature of not higher than - (minus) 6.degree. C., and thereafter dehydrating the solidified mass in vacuum until the percentage dehydration rate (i.e. the weight reduction rate of the cooled and solidified mass) reaches within the range of not less than 5 wt % and not more than 95 wt %.
According to our formerly-made proposals, an aqueous solution or an aqueous dispersion containing the polyvinyl alcohol and the water-soluble polyhydric alcohol, as described above, is submitted to prepare a gel which is excellent in elasticity and softness and is hardly frozen even when cooled in an ice box of a refrigerator.
This gel made known to the art by our preceding patent applications is clearly distinguishable from a conventional gel prepared by cooling an aqueous solution of polyvinyl alcohol, in that the former-mentioned gel is insoluble in water e.g. warm water and free from stickiness. The gel proposed by our preceding patent applications was a novel gel which was quite different from the gel prepared by the spontaneous cooling of an aqueous solution of conventional polyvinyl alcohol and also quite different from the known gel prepared through chemical treatments of conventional aqueous solution of polyvinyl alcohol.
The gel made known to the art by our preceding patent applications has a touch resembling living tissue in itself, and thus may be used as a substituent for an ice pillow or an ice bag to give a cooling medium having high elasticity. Furthermore, it is free from stickiness and is highly resilient and elastic making it possible to mold it to have a shape of a helmet which may be snugly put on a patient's head without the need of charging or enclosing it in a pouch made of a polyvinylchloride or like material for the purpose of preventing alopecia due to a harmful side effect of a carcinostatic substance.
We have investigated further on the process of the preparation of such a hydrogel, and found that a hydrogel prepared from a polyvinyl alcohol having an average molecular weight of not less than 800 may be used for the same purpose to accomplish this invention.